1. Field of the Invention
This invention relates generally to a platform for an antenna and, more particularly, to a platform for an antenna system on a satellite that supports both a receive antenna array and a transmit antenna array, where the platform is movable by a single antenna positioning device.
2. Discussion of the Related Art
Various communications systems, such as certain cellular telephone systems, cable television systems, Internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. One example is the Military Satellite Communications (MILSATCOM) satellites, known to those skilled in the art. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then re-transmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequencies, are polarized and coded. or example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna arrays, such as feed horn arrays, slot arrays, etc., which receive the uplink signals and transmit the downlink signals to the Earth. Additionally, the antenna system may employ antenna reflectors for collecting and directing both the uplink and downlink signals. In order for the antenna beams to be directed towards a particular location on the Earth, the antenna system is equipped with mechanical positioning devices that can direct the antenna beams to different locations. For example, gimbals are provided to move the antenna arrays so that the uplink signals can be received from a particular location on the Earth, and the downlink signals can be directed to a particular location on the Earth. A separate gimbal is generally provided for the transmit and receive antenna arrays so that signals can be received from and transmitted to different locations.
As is well understood in the art, it is desirable to minimize the size and weight of the satellite payload so as to conserve satellite real estate, and to reduce costs. By conserving satellite real estate, more satellite systems can be provided without increasing the satellite size. By reducing the weight of the satellite, significant cost is saved in the consumption of fuel and the like. Antenna gimbals are a primary target for reducing the weight of the satellite payload because they are heavy and sizable. Therefore, it has heretofore been known in the art to eliminate at least some of the antenna gimbals on the satellite. For example, it is known to only employ a gimbal for one of the receive or transmit arrays, and fixedly attach the other to the satellite. However, such a system is limited in its function and performance because of the directional limitations of the fixed antenna array.
It is also known in the art to use the same antenna system, including antenna array, on the satellite to receive the uplink signals and transmit the downlink signals. However, combining satellite uplink signal reception and downlink signal transmission functions in a single antenna system typically requires specialized antenna feeds capable of supporting dual frequencies and providing dual polarization. Because of this requirement, these types of antenna systems are much more complex and expensive than the single function antenna systems.
In accordance with the teachings of the present invention, an antenna system for a satellite is disclosed. The antenna system includes a receive antenna array and a transmit antenna array mounted to a common platform. An antenna positioning device is mounted to the platform and to the satellite. The positioning device causes the platform to be moved in two separate orthogonal degrees of freedom, and particularly in the longitudinal and latitudinal directions relative to the Earth. Thus, instead of employing separate positioning devices for both the receive antenna array and the transmit antenna array, a single positioning device is provided for both arrays.
The positioning device can be any suitable satellite antenna positioning system known in the art, such as uni-axial and bi-axial gimbals, wobble plates employing linear actuators, and compact small angle positioning devices using a polar coordinate system. In one embodiment, the positioning device is a gimbal device that employs a first uni-axial gimbal providing movement in one direction and a second uni-axial gimbal providing movement in a perpendicular direction. The first gimbal is mounted to a platform bracket, which is secured to the movable platform. The first gimbal is also mounted to the second gimbal, and the second gimbal is mounted to a satellite bracket secured to the satellite. Therefore, movement of the first gimbal causes the combination of the receive and transmit antenna arrays to move in one direction, and movement of the second gimbal causes the combination of the receive and transmit antenna arrays to move in the perpendicular direction.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.